that shall depend on what are the sources of income.
the process shall be that you ask for all the details regarding income and expenses made during the year. investments made etc, tax deduction details etc. self assessment tax/advance tax paid.
also ask details of past losses, copies of last years' returns and assessment orders.
Once you have all of the above details, compute the tax liability as per the tax laws for the relevant year, pick the appropriate tax return form you have to choose between ITR1, ITR2, ITR3, ITR4, ITR4S.
ask for online tax account details from the client. if the account has not been created, then create it yourself and file the tax return. after filing, download the ITRV, ask client to sign and send it to Income tax department CPC